1. Technical Field of the Invention
The present invention relates in general to the radio communications field and, in particular, to a method and system for demodulating radio signals.
2. Description of Related Art
In the digital mobile radio communications field, time dispersion is a disturbing effect which is caused when a transmitted radio signal reflects off a distant object and arrives at a receiver out of phase with the original transmitted signal. More specifically, time dispersion causes what is known as Inter-Symbol Interference (ISI), which means that consecutive symbols in the transmitted signals interfere with one another. Consequently, ISI makes it more difficult for a receiver to determine which symbol actually has been detected (or actually was sent). For example, in the digital cellular Global System for Mobile Communications (GSM), the time duration of one bit corresponds to a distance of 1.1 km. Therefore, a signal reflection from 1 km behind a receiving mobile station will have a 2 km longer path than that of the directly received signal. At the receiver, this reflection mixes the desired signal with the reflected signal (which has been delayed for a two-bit duration). In other words, the incoming bits are spread out in time at the receiver, and the adjacent bits interfere with each other. Consequently, time dispersion makes it more difficult for the receiver to determine precisely what information has been transmitted.
An existing solution for the time dispersion problem is to use an adaptive equalizer at the receiver to create a mathematical model of the transmission channel and to calculate based on the model the most probable transmitted data. In the case of the GSM and other digital cellular systems (e.g., Digital-Advanced Mobile Phone System or D-AMPS), the transmission channel is a radio air interface.
A problem with the use of an adaptive equalizer for signal demodulation is that not all radio channels are time-dispersive. Adaptive equalizers are not optimized for "flat" or non-time-dispersive radio channels. As such, the use of an adaptive equalizer for "flat" radio channels can actually be inferior to other demodulation approaches (e.g., differential detection). As described below, the present invention successfully resolves this problem.